Along with recent widespread use of photovoltaic generators, fuel cells, power storage apparatuses, and other such apparatuses for dwellings, various circuits have been proposed and provided as power-converting devices for converting outputs from direct current power supplies of such apparatuses into alternate currents. For example, JP 2014-64431 A (paragraphs [0002] to [0006], FIGS. 16 and 17, hereinafter referred to as Document 1) and JP 4369425 B2 (hereinafter referred to as Document 2) disclose a power-converting device (“multilevel power conversion device” in Document 1, “converter circuit” in Document 2) configured to generate alternate current outputs at a plurality of voltage levels by converting voltages from a direct-current voltage source.
According to the description of Document 1, the power conversion device is a five-level inverter configured to output voltages at five levels and includes two direct current capacitors, two flying capacitors, and ten switching elements. This power conversion device controls each switching element such that the voltage of each direct current capacitor is E/2 and the voltage of each flying capacitor is E/4 with a direct-current voltage E being applied to a series circuit of the two direct current capacitors, thereby outputting voltages at five levels.
The power-converting devices described in Documents 1 and 2 each have to withstand a voltage of E/4 because each switching element performs switching of the voltage E/4 in regular operation to output voltages in five levels as described above. However, a larger voltage may be applied to each switching element in a state where the flying capacitor is not charged than in the regular operation. Therefore, each switching element has to have a higher withstand voltage than a withstand voltage required in the regular operation. That is, the switching element of the power-converting device has to have a relatively high withstand voltage.